Question: Factor the following expression: $8$ $x^2+$ $39$ $x+$ $28$
Answer: This expression is in the form ${A}x^2 + {B}x + {C}$ . You can factor it by grouping. First, find two values, $a$ and $b$ , so: $ \begin{eqnarray} {ab} &=& {A}{C} \\ {a} + {b} &=& {B} \end{eqnarray} $ In this case: $ \begin{eqnarray} {ab} &=& {(8)}{(28)} &=& 224 \\ {a} + {b} &=& & & {39} \end{eqnarray} $ In order to find ${a}$ and ${b}$ , list out the factors of $224$ and add them together. The factors that add up to ${39}$ will be your ${a}$ and ${b}$ When ${a}$ is ${7}$ and ${b}$ is ${32}$ $ \begin{eqnarray} {ab} &=& ({7})({32}) &=& 224 \\ {a} + {b} &=& {7} + {32} &=& 39 \end{eqnarray} $ Next, rewrite the expression as ${A}x^2 + {a}x + {b}x + {C}$ $ {8}x^2 +{7}x +{32}x +{28} $ Group the terms so that there is a common factor in each group: $ ({8}x^2 +{7}x) + ({32}x +{28}) $ Factor out the common factors: $ x(8x + 7) + 4(8x + 7) $ Notice how $(8x + 7)$ has become a common factor. Factor this out to find the answer. $(8x + 7)(x + 4)$